This invention relates generally to line drivers and particularly to line drivers having desirable output impedance and filtering capabilities in a single amplifier stage.
DSL (digital subscriber line) is a technology for bringing high-bandwidth information to homes and small businesses over ordinary copper telephones lines. xDSL refers to different variations of DSL such as ADSL (asymmetric DSL), G.Lite DSL (ITU-T standard G-992.2), HDASL (high bit-rate DSL) and RADSL (rate-adaptive DSL).
DSL modems are typically installed in pairs, with one of the modems installed in a home (customer""s premises) and the other in the telephone company""s central office servicing that home. The pair of xDSL modems are connected to the opposite ends of the same twisted-pair transmission line.
Referring to FIG. 1 a conventional xDSL communication system 100 comprises a CO (central office) 101. The CO 101 has a plurality of xDSL modems 102 (only one shown). The xDSL modem 102 has a D/A (digital to analog) converter 104. An output of the D/A converter 104 is connected 105 to an input of an xDSL driver 106. An output of the xDSL driver 106 is connected 107 to a 4-wire input of a hybrid 108. A 4-wire output of the hybrid 108 is connected 109 to an input of an xDSL receiver 110. An output of the xDSL receiver 110 is connected 111 to the input of an A/D (analog to digital) converter 112. A 2-wire port of the hybrid 108 is connected to a transmission line 114, such as copper twisted pair.
The xDSL communication system 100 also comprises CPE (customer premises equipment) 126. The CPE 126 has an xDSL modem 122. The xDSL modem 122 has a D/A converter 124. An output of the D/A converter 124 is connected 125 to an input of an xDSL driver 126. An output of the xDSL driver 126 is connected 127 to a 4-wire input of a hybrid 128. A 4-wire output of the hybrid 128 is connected 129 to an input of an xDSL receiver 130. An output of the xDSL receiver 130 is connected 131 to an input of an A/D converter 132. The 2-wire port of the hybrid 128 is connected to the transmission line 114.
Since an xDSL modem operates at frequencies higher than the voice-band frequencies, an xDSL modem may operate simultaneously with a voice-band modem or a telephone conversation. Referring to FIG. 2, there is shown an example of a frequency spectrum plan 200 for a G.Lite DSL system on the transmission line 114 of FIG. 1. The frequency range from 0.3 to 4 kHz 202 is occupied by conventional voice communications. The frequency range from 30 to 120 kHz 204 is occupied by upstream (CPE 126 to CO 101) data transmission. The frequency range from 150 kHz to approximately 500 kHz 206 is occupied by downstream (CO 101 to CPE 126) data transmission. The upper frequency limit of the downstream data transmission is determined by the length and quality of the transmission line 114.
Referring to FIG. 3, there is shown a conventional implementation of the xDSL driver 106 of FIG. 1. The input of the xDSL driver 106 is connected 105 to an input of a bandpass filter 302. The output of the bandpass filter 302 is connected to a first non-inverting input 304 of a summation circuit 306. The output of the summation circuit 306 is connected to an input 312 of an amplifier 314. An output of the amplifier 314 is connected 316 to a first terminal of a reference resistor Re 318. A second terminal of the reference resistor Re 318 is connected to the output of the xDSL driver 107. A resistor R1 320 is connected from a second non-inverting input 308 of the summation circuit to ground 328. A resistor R2 322 is connected from output 107 of the xDSL driver 106 to the second non-inverting input 308 of the summation circuit 306. A resistor R3 324 is connected from an inverting input 310 of the summation circuit 306 to ground 328. A resistor R4 326 is connected 316 from the output of the power amplifier 314 to the inverting input 310 of the summation circuit 306.
Referring to FIG. 4, there is shown a conventional implementation of the xDSL driver 126 of FIG. 1. The topology of the xDSL driver 126 is the same as the topology of the xDSL driver 106 of FIG. 3. The differences are in the upper and lower cut-off frequencies of the filters, a bandpass filter 302 in xDSL driver 106 and bandpass filter 402 in xDSL driver 126. For example, in the case of G.Lite DSL, the lower cut-off frequency of filter 302 in xDSL driver 106 is 150 kHz, the upper cut-off frequency of filter 302 in xDSL driver 106 is 500 kHz, the lower cut-off frequency of filter 402 in xDSL driver 126 is 30 kHz and the upper cut-off frequency of filter 402 in xDSL driver 126 is 120 kHz. The gain and output impedance of xDSL driver 106 and xDSL driver 126 are substantially the same.
Unfortunately, the performance characteristics such as gain and output impedance of the conventional xDSL drivers 106, 126 are severely affected by the tolerances of the components in the positive (R1 320, 420, R2 322, 422) and negative (R3 324, 424, R4 326, 426) feedback loops and in the reference resistor (Re 318, 418). Another disadvantage of this circuit is that the active impedance generation and filtering are realized in different stages.
Thus there is a need in the industry to provide an xDSL driver that combines active impedance generation and filtering capabilities in a single amplifier stage. Furthermore, it would be advantageous to provide a line driver that would also have an independently specified gain and output impedance as well as gain that is relatively insensitive to component tolerances.
The invention may be summarized according to a first broad aspect as a line driver having an amplifier, a transformer, a reference impedance, an input impedance, a first feedback impedance and a second feedback impedance. Preferably, the amplifier is an operational amplifier connected in an inverting configuration with an input and an output. The transformer has a primary winding and a secondary winding with a ratio of 1:n. The primary winding having a first terminal connected to the output of the amplifier and having a second terminal. The secondary winding is connectable to a transmission line having a characteristic impedance. The reference impedance is connected from the second terminal of the primary winding at a junction node to a ground reference. The input impedance having one terminal connected to the input of the amplifier and another terminal connectable to a voltage source. The first feedback impedance is connected from the junction node to the input of the amplifier and the second feedback impedance is connected from the output of the amplifier to the input of the amplifier. The second feedback impedance preferably has a value equal to (Kxe2x88x921) times the value of the first feedback impedance.
In accordance with this first broad aspect of the invention, the reference impedance has a value equal to n2/K
times the characteristic impedance of the transmission line and the feedback circuit is arranged to produce a voltage at the output of the amplifier substantially equal to (K+1) times the voltage at the junction node, for a predetermined value of K. The resulting output impedance will be equal to K times the reference impedance and the gain will be equal to half of the negative of the ratio of the value of the second feedback impedance to the value of the input impedance.
According to a second broad aspect, the invention may be summarized as a line driver having a first amplifier, a transformer, a reference impedance, an input impedance, a first feedback impedance, a second feedback impedance and a second amplifier. Preferably, the first amplifier is an operational amplifier connected in an inverting configuration with a non-inverting input, an inverting input and an output and the second amplifier is an operational amplifier connected in a unity gain configuration with an input and an output. The transformer has a primary winding and a secondary winding with a ratio of 1:n. The primary winding having a first terminal connected to the output of the first amplifier and having a second terminal. The secondary winding is connectable to a transmission line having a characteristic impedance. The reference impedance is connected from the second terminal of the primary winding at a junction node to a ground reference. The input impedance having one terminal connected to the inverting input of the first amplifier and another terminal connectable to a voltage source. The first feedback impedance is connected from output of the second amplifier to the inverting input of the first amplifier and the second feedback impedance is connected from the output of the first amplifier to the inverting input of the first amplifier. The input to the second amplifier is connected to the junction node. The second feedback impedance preferably has a value equal to (Kxe2x88x921) times the value of the first feedback impedance.
In accordance with this second broad aspect of the invention, the reference impedance has a value equal to       n    2    K
times the characteristic impedance of the transmission line. The resulting output impedance will be equal to K times the reference impedance and the gain will be equal to half of the negative of the ratio of the value of the second feedback impedance to the value of the input impedance.
The invention may be summarized according to a third broad aspect as a line driver having an amplifier, a transformer, a reference impedance, an input impedance, a first feedback impedance and a second feedback impedance. Preferably, the amplifier is an operational amplifier connected in an inverting configuration with an input and an output. The transformer has a primary winding, a first secondary winding, a second secondary winding with a ratio of 1:n:m. The primary winding having a first terminal connected to the output of the amplifier and having a second terminal. The first secondary winding is connectable to a transmission line having a characteristic impedance. The second secondary winding having a first terminal connected to a ground reference and a second terminal. The primary winding and second secondary winding are arranged such that current flowing into the first terminal of the primary winding will cause current to flow into the second terminal of the second secondary winding. The reference impedance is connected from the second terminal of the primary winding at a junction node to the ground reference. The input impedance having one terminal connected to the input of the amplifier and another terminal connectable to a voltage source. The first feedback impedance is connected from the second terminal of the second secondary winding to the input of the amplifier and the second feedback impedance is connected from the output of the amplifier to the input of the amplifier. The second feedback impedance preferably has a value equal to       K    -    1    mK
times the value of the first feedback impedance.
In accordance with this third broad aspect of the invention, the reference impedance has a value equal to       n    2    K
times the characteristic impedance of the transmission line and the feedback circuit is arranged to produce a voltage at the output of the amplifier substantially equal to (K+1) times the voltage at the junction node, for a predetermined value of K. The resulting output impedance will be equal to K times the reference impedance and the gain will be equal to half of the negative of the ratio of the value of the second feedback impedance to the value of the input impedance.
The invention may be summarized according to a fourth broad aspect as a line driver having an amplifier, a transformer, a reference impedance, an input impedance, a first feedback impedance and a second feedback impedance. Preferably, the amplifier is an operational amplifier connected in an inverting configuration with an input and an output. The transformer has a primary winding, a first secondary winding, a second secondary winding with a ratio of 1:n:m. The primary winding having a first terminal connected to the output of the amplifier and having a second terminal. The first secondary winding is connectable to a transmission line having a characteristic impedance. The second secondary winding having a first terminal connected to a ground reference and a second terminal. The primary winding and second secondary winding are arranged such that current flowing into the first terminal of the primary winding will cause current to flow into the first terminal of the second secondary winding. The reference impedance is connected from the second terminal of the primary winding at a junction node to the ground reference. The input impedance having one terminal connected to the input of the amplifier and another terminal connectable to a voltage source. The first feedback impedance is connected from the second terminal of the second secondary winding to the input of the amplifier and the second feedback impedance is connected from the output of the amplifier to the input of the amplifier. The second feedback impedance preferably has a value equal to times   1  mK
the value of the first feedback impedance.
In accordance with this fourth broad aspect of the invention, the reference impedance has a value equal to       n    2    K
times the characteristic impedance of the transmission line and the feedback circuit is arranged to produce a voltage at the output of the amplifier substantially equal to (K+1) times the voltage at the junction node, for a predetermined value of K. The resulting output impedance will be equal to K times the reference impedance and the gain will be equal to the ratio of the negative of the value of the second feedback impedance to the value of the input impedance.
The invention may be summarized according to a fifth broad aspect as a line driver having an amplifier, a transformer, a reference impedance, an input impedance, a first feedback impedance and a second feedback impedance. Preferably, the amplifier is an operational amplifier connected in an inverting configuration with an input and an output. The transformer has a primary winding, a first secondary winding, a second secondary winding with a ratio of 1:n:m. The primary winding having a first terminal connected to the output of the amplifier and having a second terminal. The first secondary winding is connectable to a transmission line having a characteristic impedance. The second secondary winding having a first terminal connected to a ground reference and a second terminal. The primary winding and second secondary winding are arranged such that current flowing into the first terminal of the primary winding will cause current to flow into the first terminal of the second secondary winding. The reference impedance is connected from the second terminal of the primary winding at a junction node to the ground reference. The input impedance having one terminal connected to the input of the amplifier and another terminal connectable to a voltage source. The first feedback impedance is connected from the second terminal of the second secondary winding to the input of the amplifier and the second feedback impedance is connected from the output of the amplifier to the input of the amplifier. The second feedback impedance preferably has a value equal to             2      ⁢      K        +    1    mK
times the value of the first feedback impedance.
In accordance with this fifth broad aspect of the invention, the reference impedance has a value equal to       n    2    K
times the characteristic impedance of the transmission line and the feedback circuit is arranged to produce a voltage at the output of the amplifier substantially equal to (K+1) times the voltage at the junction node, for a predetermined value of K. The resulting output impedance will be equal to K times the reference impedance and the gain will be equal to the ratio of the value of the second feedback impedance to the value of the input impedance.
Advantageously the output impedance of the xDSL driver is specified independently from the gain or filter function of the xDSL driver. Furthermore, the gain is a simple ratio of impedances that make the gain less sensitive to component and manufacturing variations.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of the specific embodiments of the invention in conjunction with the accompanying figures.